Compression fractures of the spine are a common, painful, and debilitating complication of osteoporosis or neoplastic diseases of the vertebral bodies. In many patients, the pain is very severe and patients are unable to bear their own weight. This can require prolonged bed rest, which is known to lead to further complications including pneumonia, thromboembolism, muscle-wasting, and further bone demineralization. In addition to being painful and debilitating, compression fractures of vertebral bodies also typically result in a loss of vertebral height along the anterior margin of the involved vertebrae. This leads to an anterior wedge deformity that causes the kyphosis. Patients having compression fractures of the spine are generally treated by a procedure called percutaneous Vertebroplasty.
Vertebroplasty procedures involve the delivery of polymethylmethacrylate (PMM) cement into the body of the fractured vertebra. The procedure utilizes fluoroscopically-guided percutaneous insertion of large bore hollow cannulas into the anterior portion of the involved vertebra. The tip of the cannula is then carefully maintained within the marrow cavity. Once positioned, PPM cement, containing premixed barium to allow for fluoroscopic visualization, is instilled through the cannula. The cement material binds to the internal trabecular bone structure, resulting in stabilization of the fracture and pain relief.
Vertebroplasty has been widely used. Unfortunately, however, the procedure results in very little to no change in the vertebral height or correction of the associated kyphotic deformity. A recent study suggests that uncorrected kyphosis can lead to chronic pain despite treatment of the fracture itself. In addition, the Vertebroplasty procedure allows for very little control of the direction of the instilled PPM cement. This lack of control can lead to extra-osseous extravasation, including intravascular injection. The lack of control in the Vertebroplasty procedure can even lead to the cement straying into the spinal canal and causing nerve or spinal cord damage.
An alternate form of Vertebroplasty that addresses this kyphosis problem has been developed in the prior art as well. Similar to standard Vertebroplasty, Kyphoplasty involves the placement of a large bore cannula; however, prior to cement delivery, a balloon tamp is inserted into the vertebral body. The balloon tamp is then inflated with a contrast media that compresses the trabecular bone within the fractured vertebra, thereby creating a void. The tamp is subsequently deflated and PMM cement is instilled. Due to the void created by the balloon, the cement preferentially flows into the preformed vertebral body cavity. Additionally, the balloon may be inflated so as to controllably deflect the superior endplate in a cephalad direction. By doing so, the height of the affected vertebra may be restored, thereby correcting the kyphotic deformity associated with the compression fracture.
One potential disadvantage of Kyphoplasty, however, is that the compression of the vertebral trabecular bone by the bone tamp typically results in the formation of a pseudo-membrane around the deposited cement. The pseudo-membrane formed by Kyphoplasty prevents a true cement/trabecular bone interface from occurring. A recent study of the biomechanical integrity of such a vertebral body that has undergone a Kyphoplasty cement augmentation demonstrates that the cortical bone surrounding the deposited cement has a weaker mechanical profile than bone augmented by Vertebroplasty, where the cement is able to interdigitate with the trabecular bone. This study showed that a significant amount of vertebra height that is originally restored with Kyphoplasty is subsequently lost with mechanical loading of the vertebrae. This is further supported by a clinical study that demonstrated a progressive loss of vertebral height among patients that underwent a previous successful Kyphoplasty. As such, it appears that unless the cement is able to interdigitate with the trabecular bone, the height restoration achieved by Kyphoplasty cannot be sustained.
Another potential disadvantage of Kyphoplasty is the use of contrast material in the balloon tamp to allow for fluoroscopic visualization during the procedure. If the balloon were to rupture, the contrast material spillage may cause allergic reactions in susceptible patients.
An alternative method of displacing the trabecular bone, known as the Arcuate™ system, has been developed by Medtronics, Inc. This system utilizes a single curved blade that can be erected to perform an internal osteotomy within the trabecular bone, as opposed to the crushing method of Kyphoplasty. The blade is an arc of thin metal that acts to cut an internal osteotomy in the trabecular bone by turning of the handle, thus allowing for the PMM cement to preferentially collect osteotomized region. The single blade design is able to carve a space within the vertebra; however, it is difficult to accurately position the device under the superior endplate using fluoroscopic monitoring due to parallax and the limits of a two-dimensional projection of a single three-dimensional curve.
Furthermore, the Arcuate™ system only has two settings for the blade position, thereby limiting the adjustments available that would be necessary for the wide range of vertebral body dimensions. The larger blade height may be too large for a given vertebra, potentially resulting in the rupture of the endplate. Additionally, the two-step ratchet mechanism of the Arcuate™ system does not allow for tactile feedback to the operator. Also, the maximal height contour of the blade is at the proximal end of the curve and cannot be positioned anteriorly, where the vertebral body wedge deformity is located. Instead, the maximal height of the blade occurs in the mid to posterior aspect of the vertebral body, distant from the anterior portion of the endplate where the kyphosis is localized. As such, this system is limited to only forming an internal osteotomy within the vertebral trabecular bone, and is not suited to actual vertebral height augmentation.